Understanding Input Devices for Video Screens

Input devices are essential components of a computer system, allowing users to interact with the computer by providing data and commands. These devices translate physical actions or information from the external world into a format that the computer can understand.

The question asks for a specific type of input device that possesses two key characteristics: it can sense light, and it is used to point at locations directly on a video screen.

Analyzing the Options

• Light pen: A light pen is a computer input device shaped like a pen, used in conjunction with a computer's cathode-ray tube (CRT) display or other screen technologies. It works by sensing light emitted from the screen pixels when the pen is held against the screen. This allows the user to point to objects on the screen or draw directly onto the screen surface. This description perfectly matches the criteria mentioned in the question.
• Plotter: A plotter is an output device used to produce hard copies of graphics and drawings, typically using pens or other writing tools on paper. It does not sense light or interact with a video screen for input purposes.
• Mouse: A mouse is a widely used input device that a user moves across a flat surface to control a pointer on a display screen. While it is used for pointing, clicking, and dragging, it does not sense light from the video screen itself to determine position. Modern mice use optical or laser technology to track movement relative to the surface they are on, not the screen's light output.
• Joystick: A joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the computer. It is primarily used for controlling movement, especially in gaming and simulations. It does not sense light or point directly by interacting with the video screen surface in the way described.

Identifying the Correct Input Device

The Light pen's mechanism involves detecting the timing of the scan lines as they illuminate pixels under the pen's tip, thus determining its exact location on the screen. This direct interaction with the light output of the display is its defining feature for input.

Therefore, the only device among the options that is an input device, can sense light, and is used to point directly at spots on a video screen is the Light pen.

Revision Table: Key Input Devices

Additional Information: Light Pen Technology

The Light pen was an early input device, popular with graphics terminals. It worked by registering the time when the CRT's electron beam illuminated the phosphor at the pen's location during its scanning process. Since the electron beam scans the screen line by line from top to bottom, the timing of the light pulse detected by the pen allowed the computer system to calculate the precise coordinates being pointed at.

While less common with modern display technologies like LCDs and OLEDs, due to their different illumination methods, the Light pen was a significant pointing device in the history of human-computer interaction, enabling direct manipulation on the screen surface.